1. Field of the Invention
The present invention relates to wireless communications, and in particular, to a dynamic power control technique used in transmissions between a base transceiver station and one or more mobile stations in a wireless communication system.
2. Description of Related Art
Wireless communication networks use a number of communication schemes, such as frequency division multiple access (FDMA), code division multiple access (CDMA), time division multiple access (TDMA) schemes, and combinations of such schemes. In a TDMA scheme, over a given RF channel, each mobile station (MS) in a cell transmits and receives (to and from the base transceiver station (BTS) audio data and non-audio data packets during dedicated time slices or time slots within an overall TDMA cycle or epoch. Channels of wireless systems have varying numbers of timeslots. For example, from 1 to 8 radio interface timeslots are allocated per TDMA frame in GSM (Global System for Mobile Communication) systems, and 1 to 6 radio interface timeslots in cellular/PCS systems using a North American Dual Mode TDMA interface (hereinafter TDMA IS-136). Timeslots are shared by the active users, and uplink and downlink timeslots are allocated separately. The downlink refers to transmissions from the BTS to one or more MSs, while uplink refers to transmissions received by the BTS. The radio interface resources can be shared dynamically between speech and data services as a function of service load and operator preference. Various radio channel coding schemes are specified to allow bit rates from 9 to more than 150 kbit/s per user.
FIG. 1 illustrates a typical wireless communication network. The wireless network 10 includes a plurality of geographic sub-areas (“cells”) 12-1, . . . , 12-i. Each cell 12-1, . . . , 12-i has a corresponding base transceiver station (BTS) 14-1, . . . , 14-i for providing communication service to mobiles located therein, such as mobiles 20-1, . . . , 20j located in cell 12-1. Each of the BTSs 14-1, . . . , 14-i is connected (e.g., via a trunk line) to a mobile telephone switching office (MTSO) 16. The BTS 14-i and the mobiles 20-j communicate with each other using industry-accepted air interfaces such as the TDMA IS-136 interface for Cellular/PCS, Code Division Multiple Access interface (CDMA IS-95), GSM etc. The MTSO 16 manages communications within the network, and serves as an interface between the wireless network and a public switched telephone network (PSTN) 40, for example.
Dynamic power control is an important tool for mitigating co-channel interference, and improving the RF quality for the wireless network. The use of a downlink dynamic power control mechanism or process will allow a higher percentage of users in the wireless network to receive the desired downlink by improving the C/I (Carrier to Interference ratio) than the case of no power control for the same frequency reuse and cell coverage area. Keeping co-channel interference levels low holds the promise that high rate coding schemes can be used over the airlink. The lower interference levels achieved by power control can result in higher airlink throughputs over larger portions of the cell, potentially increasing a cell's data traffic carrying capacity. Effective power control also ensures that timeslots in wireless networks using a TDMA based air interface such as TDMA IS-136, GSM and General Packet Radio Service (GPRS) networks, do not cause unacceptable levels of interference to the users/calls in the co-channel neighboring cells.
Power control “mistakes” in wireless networks have consequences. For example, inadequate-power control may lead to poor voice quality, and dropped calls. These service disruptions are extremely annoying for users and network operators alike. Inadequate power control for mobile stations can cause high BLERs (block error rates) and degradation of uplink signal quality. Power control errors in wireless networks increase packet delays and increase packet re-transmissions due to the bit error rate (BER), resulting in decreased user throughputs. These errors generally cause service degradation rather than wholesale service disruption.
TDMA Downlink Dynamic power control (TDDPC) mechanisms allow a wireless network to adjust the power used by each BTS for transmitting the downlink RF signal. Downlink power control provides an important added benefit: transmit power used by each network BTS can be reduced to levels adequate to achieve proper airlink performance, and no higher. Transmit power can be kept as low as possible without sacrificing airlink throughput, giving users peak airlink performance without unnecessarily draining the base transceiver station's power source.